1. Field of the Invention
The present invention relates generally to a semiconductor package. More particularly, the present invention relates to a method of forming a wire loop for a semiconductor package.
2. Description of the Related Art
To connect an internal circuit of a semiconductor chip to an external circuit, a wire bonding process is performed. During the wire bonding process, electrode pads on an upper surface of a semiconductor chip are connected to leads of a lead frame through conductive wires. In more detail, the wire bonding process is performed to bond the conductive wires to the electrode pads and the leads by applying heat, ultrasonic waves, etc. while the conductive wires are being pressed against the electrode pads and the leads.
As is known in the art, a capillary equipped with one or more wires is lowered towards an electrode pad to press an end of a wire on a bonding surface so as to perform first bonding. Thereafter, the capillary is moved to a lead of the lead frame, thereby extending the wire, in order to press the opposite end of the wire against the lead so as to perform second bonding. Typically, before the first bonding is performed, a ball having a round shape is formed at the end of the wire which is to be bonded. FIG. 1 illustrates a portion of a wire 10 having a ball 11 formed at an end thereof.
To form the ball 11, the end of the wire 10 is made molten, for example by a spark discharge induced by applying a high voltage between the end of the wire 10 and a discharge electrode adjacent to the end of the wire 10. Because a high heat is generated by the spark discharge when the ball 11 is formed, a portion of the wire 10 adjacent to the ball 11 is adversely affected (e.g., annealed or otherwise hardened due to the high heat). Reference character “HAZ” denotes a heat affected zone that is caused during formation of the ball 11. A physical value (e.g., ductility) of the wire material is changed due to a thermal deformation. Thus, it is difficult to curve or bend the HAZ portion of the wire 10.
FIG. 2 is a view schematically illustrating the wire 10 having the ball 11 formed at an end thereof and which is bonded to an electrode pad 12.
The ball 11 that is formed at the end of the wire 10 is pressed on the electrode pad 12 and bonded thereto (e.g., under predetermined heat and appropriate pressure). After bonding the ball 11 to the electrode pad 12, the wire 10 is extended in a forward direction (i.e., toward a lead of the lead frame) so that the wire 10 has a predetermined loop shape. As shown in FIG. 2, reference character HL1 denotes a neck height. Neck height HL1 is a measurement of a height of the wire 10 standing upwards from the electrode pad 12, or a height of a loop from a bottom surface of the ball 11 to a top of the wire 10. As can be appreciated, it may be difficult to reduce a value of the neck height HL1 since a typical dimension of a HAZ portion of a wire is about 100 μm or more and because it may be difficult to curve or bend the HAZ portion. In this case, a height or thickness of a semiconductor package is increased. Furthermore, in this case a short-circuit may occur between upper and lower wires in the case of a multilayer multi-chip.
Accordingly, in an attempt to solve the above challenges, a wire loop having a ball neckless loop (BNL) shape as shown in FIG. 3 has been developed.
FIG. 3 is a view schematically illustrating a semiconductor package including a conventional wire loop having a BNL shape. As shown in FIG. 3, a semiconductor chip 21, which includes an electrode pad 21a on its upper surface, is disposed on a die pad 22a of a lead frame 22.
An end of a wire that includes a ball 23a is pressed on the electrode pad 21. A portion of the wire proximate the ball 23a is then pressed to fold or overlap a portion thereof so as to form a pressed part 23b on the ball 23a. The wire is extended toward a lead 22b of the lead frame 22 and another end of the wire is connected to the lead 22b to complete the formation of the wire loop 23.
However, if a difference in a height between the electrode pad 21a and the lead 22 is small or a state of the lead 22b is poor, a repelling force that is generated at the lead 22b during bonding may be applied on the wire loop 23. Thus, the wire loop 23 is repelled in a vertical direction. As a result, a height of a loop is not uniform and is difficult to be kept low.